01 - Preheat oven to 350°F. Lightly grease a 9x13-inch baking pan with non-stick cooking spray.
02 - Prepare cake batter according to package instructions. Pour batter into prepared pan and bake for 25-30 minutes, or until a wooden toothpick inserted in center comes out clean.
03 - Remove cake from oven and allow to cool in pan for 10 minutes.
04 - Using the round handle end of a wooden spoon, poke holes every 1-2 inches across the surface of the warm cake, pressing down about three-quarters of the way through.
05 - In a large bowl, whisk together instant vanilla pudding mixes and cold milk until mixture begins to thicken, approximately 2 minutes.
06 - Divide pudding mixture evenly among 4 separate bowls. Add a different gel food coloring to each bowl and stir until vibrant, uniform color is achieved.
07 - Spoon colored puddings randomly into the prepared holes, alternating colors to create a rainbow effect throughout the cake.
08 - Spread thawed whipped topping evenly over the entire surface of the cooled cake using a spatula.
09 - Sprinkle unicorn or colorful confetti sprinkles generously over whipped topping. Refrigerate cake for at least 1 hour to allow flavors to meld and pudding to set completely before slicing and serving.